As the dimensions of semiconductor devices and components continue to decrease, the demand for semiconductor wafer and photomask inspection systems exhibiting high throughput and improvements in resolution continue to increase. One manner in which higher levels of resolution are attained in semiconductor and photomask inspections systems includes the utilization of an illumination source capable of emitting shorter wavelength light.
Certain practical advantages may be achieved when illuminating a wafer or reticle with light having wavelengths at or below 400 nm. Providing suitable lasers for high quality wafer and photomask inspection systems presents a particular challenge. Conventional lasers capable of generating deep ultraviolet (DUV) light energy are typically large, expensive, and suffer from relatively short lifetimes and low average power output. In order to obtain adequate throughput and defect signal-to-noise ratio (SNR), wafer and photomask inspection systems generally require a laser based illumination source having high average power, low peak power, and relatively short wavelength.
Conventionally, the primary method for providing adequate DUV power entails converting long wavelength light to shorter wavelength light, referred to herein as “frequency conversion.” It is well known in the art that frequency conversion from longer wavelength light to shorter wavelength is often accomplished utilizing one or more non-linear optical crystals. In this context, frequency conversion requires high peak power light in order to produce a nonlinear response in a given non-linear optical crystal. To increase the efficiency of this process the longer wavelength light may be generated to have high average power, short optical pulses, and may be focused into the optical crystal. The original “longer wavelength” light is commonly referred to as “fundamental light.”
Generating light at wavelengths below 400 nm, and especially below 300 nm, is challenging. Light sources implemented in semiconductor inspection systems require relatively high powers, long lifetimes, and stable performance. Light sources meeting these requirements for advanced inspection techniques are nonexistent in the prior art. The lifetime, power, and stability of current DUV frequency converted lasers are generally limited by the implemented frequency conversion crystal and frequency conversion scheme. This is particularly true for non-linear conversion crystals exposed to DUV wavelengths, such as, but not limited to, 355, 266, 213, and 193 nm.
Many inspection applications require the frequency converted laser power or wavefront to remain stable over time. Due to degradation of the optical coatings, as a result of exposure to high power illumination, maintaining power and wavefront stability over time is challenging. This is especially true for optical coatings in the UV-DUV portion of the given frequency conversion system. These optical elements are typically not shifted so they must survive for the lifetime of the laser, typically greater than 10,000 hours and even 20,000 hours. Mirrors in the DUV below 350 nm are typically limited to power densities of approximately 100 W/cm2, and even lower for wavelengths less than 250 nm. This constraint forces optical components such as lenses and mirrors to be placed far away from the frequency conversion crystal in order to reduce the power density on the optical coatings. In the case of UV lasers with power levels greater than 0.5 W this requirement may lead to an unrealistically larger laser system.
Accordingly, it is therefore desirable to have optics in the UV-DUV portion of a frequency conversion system that can withstand very high power densities without changing over time. It is also desirable that these optics be efficient for a given wavelength range of interest, producing a minimum amount of stray light. Meeting these requirements may extend the lifetime of an implementing laser, reduce operating costs and laser maintenance time, and increase overall laser reliability.